Button to load mp3 file

The sound is linked (thanks again for the help, the website has been a big help), But I keep getting this error:

Error Scene=Scene 1, layer=1, frame=1:Line 2: Statement must appear within on handler
firstSound=new Sound();

What is the ‘handler’…like I said, I’m new at AS.
I’m not sure what this is telling me…when I preview in HTML, the sound is playing (despite the error message). But not becuase I pressed the button, it just goes on its own.

I’m probably missing something really stupid, any thoughts?

Thanks